Lead compounds and chromium compounds have been widely used as rust preventive pigments so far. These compounds have a high rust preventive effect, but, on the other hand, have toxicity problems and therefore, pollution-free rust preventive pigments are being enthusiastically developed. However, the developments of pollution-free pigments having the same durability as lead compounds or chromic acid compounds have been made only insufficiently.
There are a variety of coating systems and many coating forms corresponding to these systems. Therefore, rust preventive pigments to be formulated in these coatings differ depending on the coating form and the form of a binder resin.
When the binder resin is an alkyd resin or chlorinated rubber type resin, pollution-free rust preventive pigments reduced in surface activity are frequently used to prevent these pigments from reacting with the resin in lead suboxide, lead cyanamide and a coating. These pollution-free rust preventive pigments reduced in surface activity have an adverse effect on anticorrosive characteristics and therefore scarcely exhibit the same anticorrosive characteristics as a lead type or a chromium type. When the binder resin is an epoxy resin, polyester resin or acrylic resin, strontium chromate, lead chromate, zinc chromate, zinc phosphate or aluminum tripolyphosphate is used (see Patent Reference 1 and 2). The resin has a large effect on the cutoff of corrosive factors and the effect of the rust preventive is not significant. However, if the effect of the rust preventive pigment is significant, a highly durable coating is obtained, leading to the production of a thinner film and process savings that have been desired in coating fields.
Also, when these pollution-free rust preventive pigments contain zinc type pigments (see, Patent Reference 3), there is a problem concerning storage stability in an oil type or alkyd resin type binder. Specifically, the rust preventive pigment coagulates with time in the oil type or alkyd resin type binder. If such a coagulated anticorrosive coating composition is used, not only the condition of coating surface is impaired but also the rust preventive effect is eventually deteriorated. The reason of such a change with time is considered to be that a free fatty acid in the alkyd resin type binder adsorbs to the surface of the zinc type pigment and the zinc type pigments to which the free fatty acid is adsorbed are gradually associated and coagulated while the composition is stored.
Patent Reference 1: Japanese Patent Application Laid-Open (JP-A) No. 8-283619
Patent Reference 2: JP-A No. 2003-113482
Patent Reference No. 3: JP-A No. 60-38471.